Synthesis, structure, and properties of Ca3Co3.85M0.15O9 + δ (M = Ti–Zn, Mo, W, Pb, Bi) layered thermoelectrics

Abstract

We have prepared Ca 3 Co 3.85 M 0.15 O 9 + δ (M = Ti-Zn, Mo, W, Pb, Bi) solid solutions, investigated their crystal structure and microstructure, assessed their thermal stability in air, and measured their thermal expansion, electrical conductivity, and thermoelectric power in air at temperatures from 300 to 1100 K. The results demonstrate that the Ca 3 Co 3.85 M 0.15 O 9 + δ cobaltites are p type semiconductors and that their unit cell parameters decrease with an increase in the number of d electrons in the 3d transition metal ion and with an increase in the average oxidation state of the cobalt. Their thermoelectric power increases with increasing temperature, reaching the highest value in the Ca 3 Co 3.85 Pb 0.15 O 9 + δ solid solution: 380 μV/K at a tempera ture of 1100 K. The Ca 3 Co 3.85 Bi 0.15 O 9 + δ solid solution has the largest thermoelectric power factor among the materials studied, 206 μW/(m K 2 ) at a temperature of 1100 K, which is twice the power factor of the unsub stituted calcium cobaltite Ca 3 Co 4 O 9 + δ .

Prospects for Electrical Performance Tuning in Ca3Co4O9 Materials by Metallic Fe and Ni Particles Additions

“…It has been reported that partial substitutions with Fe and Ni for Co in Ca 3 Co 4 O 9 may increase the electrical conductivity by increasing the holes concentration from the conductive layers, provided that Fe and Ni take the usual oxidation states (2+) and/or (3+) [ 33 , 46 ], smaller than that of the mean Co valence responsible for the good electrical conductivity from this material, between (3+) and (4+) [ 29 ]. This fact was observed for small substitutions.…”

Thermoelectric Properties of Inhomogeneous Ceramics Based on the Layered Calcium Cobaltate

“…The values of electrical conductivity and coefficient of thermo-EMF of ceramics, in the whole, were increased at increasing of the cobalt oxide content in the samples ( Table 5). The values of the apparent activation energy of electrical conductivity of the samples of A serie varied within 0.069-0.081 eV, which is typical for ceramics based on the layered calcium cobaltate [12,14,16] (excluding the samples having Ca 3 Co 3.8 O 9+δ , Ca 3 Co 4 O 9+δ composition, which had been sintered at 970°С, whose E A values were equal to 0.274 and 0.125 eV respectively and were abnormally high, which was probably due to the peculiarities of their microstructure).…”

“…The values of the apparent activation energy of electrical conductivity of the samples of N serie varied within 0.050-0.072 eV, which is typical for ceramics based on the layered calcium cobaltate [12,14,16], slightly depended on the cationic compostiton of the samples and temperature of their sintering, and were less than for the samples which had been sintered in air or in oxygen.…”

“…Samples which were sintered in oxygen were also p-type (S > 0) semiconductors (∂σ/∂T>0) ( Figure 5, middle block), although their electrical conductivity values at temperature gaining increased less significant than for the samples, sintered in air. (1, 2, 7, 8, 13, 14), Ca 3 Co 3.8 O 9+δ (3,4,9,10,15,16), and Ca 3 Co 4.2 O 9+δ (5,6,11,12,17,18) ceramics, annealed in air (1 -6), oxygen (7 -12) and nitrogen (13 -18) at 890°C (13), 920°C (1,3,5,15,17), 970°C (2,4,6,7,9,11,14,16,18), and 1020°C (8, 10, 12) The highest electrical conductivity was demonstrated by ceramics having Ca 3 Co 4 O 9+δ composition which had been sintered at 1020°C, and the highest value of the thermo-EMF coefficient was observed for ceramics of Ca 3 Co 3.8 O 9+δ composition which had been sintered at 970°C ( Table 5). It is interesting to note that the values of the thermo-EMF coefficient of ceramics containing an excess of both calcium oxide (Ca 3 Co 3.8 O 9+δ ) and cobalt oxide (Ca 3 Co 4.2 O 9+δ ) were higher than for the basic Ca 3 Co 4 O 9+δ phase with stoichiometric relation of cations (Table 5).…”

“…The indicated drawbacks are largely absent for oxide thermoelectrics, including materials based on the layered calcium cobaltate Ca 3 Co 4 O 9+δ , which in recent years are considered as a promising basis for the development of p-branches of high-temperature thermoelectric generators [2,3]. The functional (thermoelectric) characteristics of ceramics based on the Ca 3 Co 4 O 9+δ can be essentially improved by using instead of the traditional ceramic methods the "soft" low-temperature synthesis methods [4,5], using special techniques for sintering ceramics -hot pressing [6], spark plasma sintering [7][8][9], and also by partial substitution of calcium ions in it by bismuth [3,[10][11][12] or rare earth elements ions [3,13,14] or of cobalt ions by 3d-metal ions [3,15,16]. It was shown in [11,[17][18][19] that thermoelectric characteristics of oxide ceramics can be improved by creating phase inhomogeneity in it.…”